I think the kind of background applications people wanted to write involved a temporal resolution finer than five minutes.

But to answer your question: Yeah, it really would eat battery. If my iPod Touch is any indication of battery life and its usage with wifi, what you describe would mean that if I charged the player before I went to bed, I'd need to charge it again before I could it to listen to music during my morning jog.

I know that your particular application wouldn't be left running while you slept. But some of the background apps people want to make *would*. Hence it's a big deal.